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Abstract

Galls induced by insects are specialized plant tissues thought to provide a suitable microclimate and high-quality food for insect development. Galls are also hypothesized to provide protection from predators, and particularly from parasitoids, because larger galls may be too deep for parasitoid oviposition (enemies hypothesis). However, galls may actually increase the risk of parasitism by making the location of gallformers more apparent (apparency hypothesis). Rhopalomyia pomum (Diptera: Cecidomyiidae) forms soft, lobed galls on big sagebrush (Artemisia tridentata: Asteraceae). The volume of the largest galls can be 600× that of the smallest. Here I explore the relationships of the number of lobes per gall and gall volume with the emergence of R. pomum and its parasitoids. I collected 159 galls from 4 locations in southern Utah in spring 2002, measured them, and monitored emergence from each. Lobing was not related to midge emergence (F1,150 = 2.35, P = 0.13) but was positively associated with parasitoid emergence (F1,150 = 15.27, P < 0.001), suggesting that early season parasitoids attacking before gall development may contribute to lobe formation by disrupting cues from eggs or larvae to the plant, or that flies in lobed galls are more accessible to oviposition by late season parasitoids. More midges emerged from larger galls than from smaller galls (F1,150 = 22.0, P < 0.001), but gall size was not related to parasitoid emergence (F1,150 = 0.3, P = 0.6), providing no support for either the enemies or apparency hypothesis.

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